End-Functionalization of Diarylethene for Opto-Electronic Switching with High Fatigue Resistance

Article abstract of DOI:10.1021/acs.chemmater.0c04219

A molecular and synthetic approach to strengthen the switching performance of diarylethene (DAE)-based organic transistors is proposed. By tuning the length of alkyl side chains of the biphenyl unit attached to 1,2-bis(5-biphenyl-2-methylthien-3-yl)perfluorocyclopentene (DAE), we show that the molecular environment for reversible photoisomerization of DAEs can be optimized. Four different DAEs are synthesized with different alkyl chains (DAE_C0, DAE_C1, DAE_C6, and DAE_C10), and ITIC is chosen to construct a semiconductor matrix to maximize the quantum yield of photoconversion considering the complementary absorption range of both materials. From photophysical, structural, and morphological analyses, the longer alkyl chains inhibit intermolecular aggregation between DAEs and allow more hydrophobic surface properties of DAEs, thus improving molecular miscibility with ITIC. The improved molecular compatibility of DAEs with ITIC makes the overall bulk heterojunction film amorphous, allowing more free volume for reversible photoisomerization. Consequently, DAE_C6 exhibits the maximum quantum yield for both photocyclization and photocycloreversion, enabling high light-controlled on/off ratios in photoswitchable transistors. Furthermore, the exceptionally high DAE_C6 quantum yield enables robust fatigue resistance under repeated photoswitching with only a 30% decrease in the on/off ratio after 100 cycles. Overall, this work shows that not only the energy level but also the molecular compatibility can endow significant switching performances for molecular switches.

Full text of DOI:10.1021/acs.chemmater.0c04219

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Article
End-Functionalization of Diarylethene for Opto-Electronic Switching
with High Fatigue Resistance
Syed Zahid Hassan,§ Seong Hoon Yu,§ Chan So, Dohyun Moon, and Dae Sung Chung*
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ABSTRACT: A molecular and synthetic approach to strengthen the
switching performance of diarylethene (DAE)-based organic transistors is
proposed. By tuning the length of alkyl side chains of the biphenyl unit
attached to 1,2-bis(5-biphenyl-2-methylthien-3-yl)perfluorocyclopentene
(DAE), we show that the molecular environment for reversible photo-
isomerization of DAEs can be optimized. Four different DAEs are synthesized
with different alkyl chains (DAE_C0, DAE_C1, DAE_C6, and DAE_C10),
and ITIC is chosen to construct a semiconductor matrix to maximize the
quantum yield of photoconversion considering the complementary
absorption range of both materials. From photophysical, structural, and
morphological analyses, the longer alkyl chains inhibit intermolecular
aggregation between DAEs and allow more hydrophobic surface properties of DAEs, thus improving molecular miscibility with
ITIC. The improved molecular compatibility of DAEs with ITIC makes the overall bulk heterojunction film amorphous, allowing
more free volume for reversible photoisomerization. Consequently, DAE_C6 exhibits the maximum quantum yield for both
photocyclization and photocycloreversion, enabling high light-controlled on/off ratios in photoswitchable transistors. Furthermore,
the exceptionally high DAE_C6 quantum yield enables robust fatigue resistance under repeated photoswitching with only a 30%
decrease in the on/off ratio after 100 cycles. Overall, this work shows that not only the energy level but also the molecular
compatibility can endow significant switching performances for molecular switches.
introduction of semiconducting properties to DAEs.7,21−24
Unfortunately, this approach failed to achieve high perform-
ance in terms of both charge carrier mobility and current
INTRODUCTION
Endowing more and more functionality to the given device
dimension can be a smart solution for increasing the integrated
■
modulation. Introducing a covalent bond or a supramolecular
link between DAEs and the organic semiconductors (OSCs) is
another possible approach that can guarantee the maximum
areal exposure of DAE to OSCs.25 However, this approach has
the limitation of hampering the molecular arrangement of the
semiconductor by changing either the lamellar distance or the
π−π stacking distance. Blending DAEs with OSCs can be the
simplest method to not sacrifice the properties of the
semiconductor, and very successful demonstrations were
circuit density without sacrificing the physical volume of the
unit device.1,2 Embedding a molecular switch in the semi-
conductor matrix can be a good method to bring such
multifunctionality to the unit device, if the selected molecular
switch can tune any device electrical properties in a reliable
and reproducible way while maintaining the properties of the
semiconductor.3,4 From this point of view, photochromic
molecules which use nondestructive light as a stimulus have
been vastly applied to various electronic devices, such as field
effect transistors (FETs),5−9 memory devices,10−13 and
bioimaging sensors.14−16 Photochromic molecules can undergo
reversible isomerization in response to light stimuli by
producing two interchangeable metastable states. To date,
there have been several kinds of photochromic molecules, such
as diarylethenes (DAE), azobenzenes, spiropyrans, stilbenes,
and fulgides. Among these, DAE is the most promising for
embedded molecular switches in thin film devices due to its
(1) relatively small volumetric change between two isomers,
(2) thermal stability of each metastable state at ambient
conditions, and (3) fatigue resistance even after many cycles of
light irradiation.17−20 Photo-induced switching behaviors of
thin film organic electronic devices have been reported by
several approaches. The most direct approach is the
3,4,26−31
̀
introduced by Samori et al. for many OSCs.
In the
case of the bulk heterojunction (BHJ) system, the key to the
success of electrical switching is how much of the preferred
molecular environment can be allowed to each single DAE
molecule so that photocyclization and photocycloreversion can
be successfully achieved. Therefore, it is important to
Received: October 30, 2020
Revised: December 18, 2020
Published: December 29, 2020
https://dx.doi.org/10.1021/acs.chemmater.0c04219
© 2020 American Chemical Society
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a
Scheme 1. Synthetic Routes for Biphenyl Substituted DAE Derivatives
a
(1) n-BuLi, tetrahydrofuran (THF), −78 °C, 1 h; 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, −78 °C to rt, 1 h; (2) 1-bromo-4-
iodobenzene, Pd(PPh3)4, Na2CO3, reflux; (3) n-BuLi, THF, −78 °C, 1 h; 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, −78 °C to rt, 1 h;
(4) 3,5-dibromo-2-methylthiophene, Pd(PPh3)4, Na2CO3, reflux; and (5) n-BuLi, THF, −78 °C, perfluorocyclopentene, 1 h.
Figure 1. (a) Chemical structures of synthesized biphenyl substituted DAE derivatives, DAE_C0, DAE_C1, DAE_C6, and DAE_C10, in their
open- and closed-ring isomers. Reversible photoisomerization of the open-ring isomer to the closed-ring isomer was achieved by irradiating with
UV (375 nm) or visible light (520 nm), as displayed in the figure by the arrow, (b) Chemical structure of the OSC, ITIC, (c) UV−visible
absorption spectra of ITIC and DAE_C6 in their open- and closed-ring isomers, showing the optical transparency of ITIC in the region of 250−
450 nm as compared to the DAE_C6 open-ring isomer, (d) energy level diagram indicating the hole trapping mechanism between the HOMO
levels of ITIC and DAE in their open- and closed-ring isomers (energy levels were defined by CV measurements, see Supporting Information), and
(e) Device geometry and conceptual image of photoswitching measurements.
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Figure 2. UV−visible absorption spectra of the ITIC/DAE BHJ films and their PSS. (a) ITIC/DAE_C0 BHJ film, (b) ITIC/DAE_C1 BHJ film,
(c) ITIC/DAE_C6 BHJ film, and (d) ITIC/DAE_C10 BHJ film. Reversible photoisomerization was carried out by UV light irradiation (375 nm)
and visible light irradiation (520 nm). The UV irradiation time was fixed at 3 s and the visible light irradiation time was fixed at 60 s, which showed
no further change in the drain current upon further irradiation at the same wavelength.
determine the effects of the DAE molecular structure on the
switching performances of the final device.
for DAE_C0, DAE_C1, DAE_C6, and DAE_C10, respec-
tively. A detailed synthetic method for DAE derivatives is
presented in the Supporting Information.7 Biphenyl-substi-
tuted DAE, rather than phenyl-substituted DAE, was chosen
for the larger π-electron delocalization to achieve a better
intermolecular interaction with the OSCs. In fact, DAE with a
biphenyl moiety showed semiconductor properties, which is
one of the few reported semiconducting DAE derivatives.7
Perfluorinated DAEs were synthesized because previous works
showed that cyclopentene fluorination can make the DAE
switching ability faster by a factor of 4−5 with respect to the
nonfluorinated analogue.32−35 3,9-bis(2-methylene-(3-(1,1-di-
cyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphen-
yl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]-
dithiophene (ITIC) was chosen as the π-conjugated matrix
OSC (Figure 1b),36−39 because it is a molecular semi-
conductor and a lower enthalpy of mixing with DAE can be
expected when both the molecules have similar surface
energies and a higher entropy of mixing with DAE because
both of them are small molecules with a molecular weight less
than 1500 D. In this case, the change in the Gibbs free energy,
ΔG = ΔH − TΔS, can be maximized to a negative value, which
can ensure better miscibility between the semiconductor and
the molecular switch.40 Additionally, ITIC was selected as the
OSC as it is amorphous until thermally not treated and a
suitable candidate for hole trapping as its highest occupied
In this study, to simultaneously maximize the photo-
cyclization/photocycloreversion quantum yield (Q.Y.), and
thus the switching performances in FETs, four biphenyl
substituted DAE derivatives with different end alkyl chain
lengths, namely, DAE_C0, DAE_C1, DAE_C6, and
DAE_C10, were systematically designed and synthesized.
Concurrently, by varying the alkyl chain, the study attempted
to realize an optimal DAE surface energy, which matches that
of the π-conjugated OSCs, both of which can be expected to
increase photocyclization. Moreover, the study shows that the
longer alkyl chains of DAEs can practically reduce the
crystalline ordering of the OSCs, allowing more free volume
for photocycloreversion. Systematic analyses, to show the
morphological/structural effects of DAEs with various alkyl
chain lengths, are introduced by means of UV−visible
absorption, water contact angle, grazing incidence X-ray
diffraction (GIXD), atomic force microscopy (AFM), and
differential scanning calorimetry (DSC), followed by detailed
discussions.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Scheme 1 and Figure 1a
summarize the synthetic procedures and chemical structures
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molecular orbital (HOMO) level is located between the
HOMO level of DAE-o and DAE-c. Furthermore, ITIC
possesses optical transparency for the absorption range of
DAEs synthesized in this study for open isomers (Figure 1c).
This implies that a maximized Q.Y. for photocyclization can be
expected even though they are blended within the semi-
conductor matrix. To propose optically switchable transistors
using synthesized DAEs, the energy levels of the DAEs were
confirmed by cyclic voltammetry (CV). As summarized in
Figure 1d and Table S1, the HOMO levels of DAE_C0,
DAE_C1, DAE_C6, and DAE_C10 had similar values of
∼−5.8 eV in the open-ring isomer and ∼−5.2 eV in the closed-
ring isomer of the synthesized DAEs. Therefore, the increase of
the linear alkyl chains at the end of the DAE backbone had a
minor effect on the molecular orbitals owing to the minor
influence in conjugations; thus, all DAEs showed almost
similar optical band gaps. The HOMO levels of these DAEs
can induce effective hole carrier trapping by a difference of
∼0.3 eV in the closed form, compared to the HOMO level of
ITIC (∼−5.5 eV). In contrast, the open-ring isomer of DAEs
has a much deeper HOMO level than that of ITIC and thus
does not act as trapping sites for the ITIC hole anymore.
Therefore, it can be concluded that DAEs and ITIC can be an
effective material combination for photoswitchable optoelec-
tronic devices. Figure 1e schematically depicts the geometric
structure of the used ITIC/DAE transistor presenting the
bottom gate/bottom source-drain electrode configuration.
Photochemical Studies. To study the photoisomerization
behavior of the synthesized series of DAEs, the UV−visible
absorption spectra of DAEs in the open-ring isomer, namely,
DAE_C0-o, DAE_C1-o, DAE_C6-o, and DAE_C10-o, and
the closed-ring isomer, namely, DAE_C0-c, DAE_C1-c,
DAE_C6-c, and DAE_C10-c, were obtained in solution,
film, and their BHJ film with ITIC. Figure S1 shows the
UV−visible absorption spectra of the DAE derivatives in
CHCl3 solution. The synthesized DAEs possessed similar
absorbance features in both the open-ring and closed-ring
isomers as they all have the same π-conjugated structure
throughout their molecular backbone. Concurrently, a slight
but clear bathochromic shift of both the open- and closed-ring
isomers was visible as the alkyl chain length increased, which
can be ascribed to the electron donating effect of the longer
alkyl chain on the conjugated DAE structure. Figure S2
summarizes the UV−visible absorption spectra of the DAE
derivatives in the film state, which shows that photo-
isomerization is also successful in the film state. To
photocyclize DAE-o (open-ring isomer) to DAE-c (closed-
ring isomer), the DAE sample was irradiated with UV light (λ
= 375 nm) which generated a new absorption band between
450 and 650 nm. Figure 2a−d depicts the UV−visible
absorption spectra of the ITIC/DAE BHJ film, before and
after UV light irradiation, where the DAE amount in the BHJ
film was 40 wt %. Note that a 60%:40% = ITIC/DAE
composition by weight was selected in the BHJ film, as it was
found that 40 wt % of DAE resulted in the best performance
FETs in our studies (Figure S3). Figure S4 depicts the
differential changes in the absorption spectra of the ITIC/DAE
BHJ film upon UV light irradiation, clearly showing an increase
in absorption between 450 and 650 nm and a concomitant
decrease in absorption between 275 and 395 nm, indicating
the successful conversion of the open-ring isomer to the
closed-ring isomer. Overall, the λmax of DAE in the BHJ film
was blue-shifted when compared to the pristine DAE film,
implying that the DAEs and ITIC were well intermixed (Table
1). Upon continuous irradiation with UV light, the absorption
Table 1. Absorption Characteristics and Photoreactivity of
DAEs in the ITIC Matrix
λmax
a
b
c
open
closed
form
conversion
ratio (%)
d
e
form
ϕUV
ϕvis
ITIC/DAE_C0
316
599
587
599
590
48
47
58
0.42
0.41
0.68
0.37
0.50
0.51
0.53
0.38
film
ITIC/DAE_C1
film
ITIC/DAE_C6
film
ITIC/DAE_C10
film
316
319
316
53
a
b
Absorption maxima for the open-ring isomer. Absorption maxima
c
for the closed-ring isomer. Amount of the closed-ring isomer in PSS
d
upon UV light irradiation (375 nm, 0.15 mW/cm2). Q.Y. of the
e
open- to closed-ring isomer with an experimental error of 10%. Q.Y.
of the closed- to open-ring isomer with an experimental error of 10%.
band at λmax for the closed form continuously increased,
leading to the generation of a photostationary state (PSS)
which is defined as the equilibrium between DAE-o and DAE-
c. Photocycloreversion was conducted by irradiating the DAE
sample in the PSS with visible light (λ = 520 nm). Note that a
green laser with a wavelength of 520 nm was chosen instead of
longer wavelength light sources as the ITIC band I absorption
sharply increases after ∼550 nm. The absorption maxima for
the open- and closed-ring isomers and the PSS and Q.Y. for
photocyclization and photocycloreversion of DAE in the BHJ
film are listed in Table 1. The detailed methods used for the
estimation of PSS and Q.Y. are described in the Supporting
Information, which followed methods detailed in previous
studies.41−45 Table S2 summarizes the photoconversion ratio
and Q.Y. of DAE_C0, DAE_C1, DAE_C6, and DAE_C10 in
the solution and thin film and Table 1 summarizes those in the
BHJ films with ITIC. The photoconversion ratio in the
solution state exceeded 80% for all the synthesized DAEs,
which is caused by rapid conformational changes in the parallel
and antiparallel forms of the DAE-o derivatives in solution
state at room temperature, as reported earlier.42 Conforma-
tional change of the parallel and antiparallel form of DAEs is
not possible in the thin film state. That is, only the photoactive
antiparallel form of DAE photocyclizes to the closed-ring
isomer and thus the PSS in the thin film is lower than that in
the solution state. As summarized in Table 1, both the PSS and
ΦUV of DAEs in the BHJ film increased as the length of the
alkyl chain increased, presumably because of the bulkier alkyl
chain, which can (1) reduce the intermolecular π−π
interaction between DAEs and thus induce more free volume
for conformational change and (2) increase the molar portion
of the photoactive antiparallel conformer, as DAEs with a bulky
alkyl chain length prefer the antiparallel conformer.46 The
exception observed for DAE_C10 had the lowest photo-
cyclization and photocycloreversion Q.Y. which can be
ascribed to a very large alkyl chain volume, which requires a
very high volumetric change for photoisomerization.4 Interest-
ingly, the Q.Y. for photocycloreversion also increased as the
alkyl chain length was increased. This can be related to
volumetric expansion during photocycloreversion. Because a
larger DAE volume can ensure a larger space in the open-ring
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isomer, and as the ITIC matrix becomes amorphous when
mixed with DAE, as discussed in the GIXD section, it provides
a flexible inner space for the closed-ring isomer to easily
recover back to its open-ring isomer form. For example,
DAE_C6 showed almost twice the difference in unit cell
volume between the two photoisomers, as confirmed by single
crystal XRD (Table S3). The relatively high DAE_C6 Q.Y. is
also very important for electrical fatigue resistance because a
high Q.Y. minimizes the cumulative portion of the undesired
conformational isomer.
Studies on Molecular Compatibility. To further
elucidate the origin of the highest photocyclization and
photocycloreversion Q.Y. of DAE_C6, the molecular compat-
ibility of the synthesized DAEs with ITIC was studied by
means of surface energy, GIXD, AFM, and DSC analyses. To
investigate the effect of different alkyl chains on the surface free
energy of DAEs, the water contact angle for each pristine ITIC,
DAEs, and their BHJ films was measured. Because DAEs were
dissolved in diiodomethane, the calculation of the surface free
energy could not be completed. In the case of the pristine ITIC
film, the average water contact angle was 95.6 0.7°, and there
was a slight change in the contact angle after exposure to UV
light, which also highlights the photostability of ITIC along
with the negligible change observed in the UV−visible
absorption spectrum of ITIC before and after UV light
irradiation, as shown in Figure S5. In the case of the pristine
DAE films, the overall hydrophobicity was enhanced when the
DAE molecules photoisomerized from open- to closed-ring
isomers by UV exposure, such that the water contact angle for
DAE_C0 changed from 79.5 1.1 to 91.3 1.0°, DAE_C1
changed from 79.8 0.6 to 93.3 1.0°, DAE_C6 changed
from 92.4 0.7 to 101.0 0.9°, and DAE_C10 changed from
91.4 0.3 to 98.1 0.5° (Figure S6).47,48 This phenomenon
is due to the denser intermolecular packing of DAE molecules
in the closed form, as the overall cell volume decreased when
the DAE open-ring isomer changed to the closed-ring isomer,
as studied by single crystal XRD (Figure S7a−d and Table
S3).49 Additionally, as the DAE alkyl chain length was
increased, the DAE thin film became more hydrophobic
leading to a higher water contact angle. Because the DAEs and
their BHJ solutions with ITIC were kept in dark conditions, it
can be assumed that most DAEs remain as open-ring isomers
when their solutions were deposited onto the substrate for spin
coating. Therefore, a similar hydrophobicity of the open-ring
isomer is needed for better miscibility with ITIC. The
difference in the water contact angle of DAE_C0, DAE_C1,
DAE_C6, and DAE_C10 with the ITIC was approximately
16.1 1.1, 15.8 0.7, 3.2 0.7, and 4.1 0.7°, respectively.
This implies that the miscibility of ITIC/DAE_C0 and ITIC/
DAE_C1 was poorer than ITIC/DAE_C6 and ITIC/
DAE_C10.50 In this context, DAE_C6 with a water contact
angle of 92.4 0.7° should have minimized the surface free
energy difference with ITIC, which has a water contact angle of
Figure 3. Water contact angle of ITIC/DAE_C0, ITIC/DAE_C1,
ITIC/DAE_C6, and ITIC/DAE_C10 BHJ films, where DAE is in
both open- and closed-ring isomer forms. Photoisomerization of the
open-ring isomer to the closed-ring isomer was achieved by irradiating
the samples with UV light. The boxes in the plot represent the
standard deviation of the measured water contact angle. The water
contact angle values were obtained from five independent points at
room temperature. The actual water droplets on the ITIC/DAE BHJ
film surface are shown in the upper part of the figure.
concluded that DAE_C6 can be an optimal counterpart to
ITIC by means of molecular compatibility. In a similar context,
we can also explain the lowest switching performance in the
case of DAE_C0.
Two-dimensional GIXD analyses were performed to
examine the crystalline ordering changes of the ITIC/DAE
BHJ films when the DAE alkyl chain length was changed.52,53
Note that post thermal annealing was not employed for all the
ITIC/DAE BHJ films. Figure 4a−f shows the GIXD pattern
images, as well as out-of-plane line cut profiles of the ITIC and
ITIC/DAE BHJ films.36,54 In-plane line cut profiles of the
ITIC/DAE BHJ films are summarized in Figure S8a. The
pristine ITIC film shows weak diffraction features with a small
diffraction peak at qz = 0.42 Å−1 (100) because no thermal
treatment was used. When mixed with DAEs, the ITIC/DAE
BHJ films exhibited considerably less featured diffraction
patterns. Overall, the ITIC lamellar diffraction peak (100)
gradually broadened as the DAE alkyl chain length increased.
The lamellar spacing increased to 15.93 and 16.46 Å in the
ITIC/DAE_C0 and the ITIC/DAE_C1 BHJ films, respec-
tively, as compared to the ITIC lamellar spacing of 14.65 Å,
which suggest that DAE molecules intercalated within the
ITIC crystalline domains. The lamellar spacing for ITIC/
DAE_C6 and ITIC/DAE_C10 could not be obtained as the
GIXD patterns became completely featureless. Additionally,
the π−π spacing for ITIC/DAEs could not be obtained
because of very weak diffraction in that region (Table S4). This
was somewhat expected because the DAEs studied herein are
not crystalline as they are simply cast on the substrate (Figure
S8b−d). However, it forms a crystalline packing when
assembled as a single crystal in the solution, as shown by the
single crystal structure (Figure S7). Clearly, it can be
concluded that the overall crystalline ordering of the ITIC/
DAE BHJ film worsened as the alkyl chain length of the
synthesized DAEs increased. Additionally, we could obtain
∼95.6
0.7°. This explains the optimized miscibility of
DAE_C6 within the ITIC matrix, allowing the hydrophobic
alkyl chains of DAE_C6 to be exposed to the surface more
efficiently, which explains the higher hydrophobicity of the
ITIC/DAE_C6 BHJ film as compared to the pristine ITIC
film. Interestingly, the ITIC/DAE_C10 BHJ film showed a
slightly lower or similar water contact angle to that of the
ITIC/DAE_C6 BHJ film, as shown in Figure 3, which could
be caused by the random molecular packing of DAE_C10
because of very long alkyl chains.51 Therefore, it was
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Figure 4. Two-dimensional GIXD images of (a) ITIC, (b) ITIC/DAE_C0 BHJ film, (c) ITIC/DAE_C1 BHJ film, (d) ITIC/DAE_C6 BHJ film,
(e) ITIC/DAE_C10 BHJ film, and (f) corresponding line cut profiles in the out-of-plane direction where the right side of the line cut profile shows
a magnification between 0.3 and 0.5 Å−1 and where the dashed line shows the shift in the (100) peak in the ITIC/DAE BHJ films when the alkyl
chain length was increased (black dashed line = ITIC, red dashed line = ITIC/DAE_C0, and blue dashed line = ITIC/DAE_C1).
only negligible change in in-plane or out-of-plane line cut
profile for ITIC/DAE BHJ films upon UV exposure which
suggest that the films remain mostly amorphous regardless of
photoisomerization (Figure S9).
type behavior of the ITIC/DAE_C6 FET performance after
UV and visible light irradiation, which is obvious as the lowest
unoccupied molecular orbital (LUMO) levels of both the
open- and closed-ring isomers of DAE are shallower than that
of ITIC. The switching performance of all the FETs is defined
by the following equation
Moreover, the melting point obtained from DSC showed a
slightly lower melting point in the case of the ITIC/DAE_C6
BHJ as compared to the pristine ITIC, as shown in Figure
S10a,b, which also implies that the ITIC was well mixed with
DAE_C6.3,55,56 Additionally, the morphological differences
between the ITIC/DAE_C6 BHJ film and the pristine ITIC
were also investigated by means of AFM, as summarized in
Figure S11. The pristine ITIC exhibited a granular
morphology, a feature of small crystalline domains, while all
the different weight ratio (10, 20, 40, and 60 wt %) samples of
the ITIC/DAE_C6 BHJ films exhibited a featureless
morphology with considerably lower root mean square (rms)
roughness. The roughness of the ITIC/DAE_C6 BHJ films
decreased as compared to the pristine ITIC film from ∼0.55 to
∼0.25 nm as the DAE content increased. Interestingly, the rms
roughness value was also the lowest in the case of the 40 wt %
ITIC/DAE_C6 BHJ film (Figure S12), implying the optimized
miscibility between ITIC and DAE_C6. Additionally, the
change in surface morphology before and after UV exposure
was investigated; however, the phase separation phenomenon
could not be found in either condition.
ON/OFF ratio = |IVIS|/|IUV
|
where IVIS is the initial drain current or after visible light
irradiation and IUV is the drain current after UV light
irradiation from the IDS versus VG plot where VDS and VG
was fixed to −20 and −30 V, respectively. Figure S3 shows the
switching performance of the ITIC/DAE_C6 FETs when 10,
20, 40, and 60 wt % DAE_C6 were added. The ITIC/
DAE_C6 FET with 40 wt % of DAE showed the best
switching performance in terms of drain current. All the DAEs
enabled a successful current modulation between the on/off
states with an on/off ratio exceeding 20; however, the degree
of current modulation between the on/off states became
highest in the case of ITIC/DAE_C6 with the maximum on/
off ratio of 50, consistent with the observation of the maximum
ΦUV and Φvis. Additionally, we have calculated the hole
mobilities for ITIC/DAE_C0, ITIC/DAE_C1, ITIC/
DAE_C6, and ITIC/DAE_C10 OFETs in their open and
closed form as summarized in Table S5.57,58 From the
threshold voltage of the transfer curve, the trap density can
be estimated using the following equation
Switching Performance. To elucidate the switching
behavior of the synthesized DAEs, bottom gate/bottom
contact FETs were fabricated for all the ITIC/DAE BHJ
films. ITIC exhibits a typical ambipolar behavior as shown in
Figure S13a,b. In this study, the p-type nature of ITIC was
utilized to induce the hole trapping mechanism by the DAE
molecules. Figure S14 shows that there was no change in the n-
CiV
e
th
Ntr =
where Ci is the capacitance of a dielectric layer, Vth is the
threshold voltage extracted from transfer characteristics, and e
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Figure 5. Photoswitching performance of ITIC/DAE FET devices. (a) Transfer curves of the ITIC/DAE_C6-based FETs for alternate steps of UV
(375 nm) and visible (520 nm) light irradiation up to 10 cycles. (b) Continuous switching behavior of the ITIC/DAEs based FETs after irradiation
with UV (375 nm, 0.15 mW/cm2) and visible light (520 nm, 0.15 mW/cm2) of up to 10 cycles. The connecting lines are used as guides. Each
current value was extracted from the transfer curves repeated up to 10 cycles. (c) Reversible modulation of the drain current of the ITIC/DAE_C6
based FETs irradiated with UV (375 nm, 0.15 mW/cm2) and visible light (520 nm, 0.15 mW/cm2) up to 100 cycles. The UV irradiation time was
fixed at 3 s, and the visible light irradiation time was fixed at 60 s, with no further change in the drain current being observed upon further
irradiation at the same wavelength. All single points of the ID values were extracted from the transfer curves at VG = −30 V and VDS = −20 V. The
enlarged figure of switching behavior in the ITIC/DAEs based FETs under UV (375 nm) irradiation extracted from Figure 5b can be found in
Figure S15 for better readability.
is the electron charge. Because of photoisomerization, ITIC/
DAE_C6 FETs showed an increase in the concentration of
trap states from ∼1.45 × 1012 to ∼2.27 × 1012 cm−2 for pristine
and UV exposure condition, respectively (Table S6).57
The electrical fatigue resistance of the best performing
ITIC/DAE_C6 FET was determined, and it showed only a
10% decrease in the drain current after 10 on/off repetitive
steps, while all other ITIC/DAE FETs exhibited considerably
worse fatigue resistance, which again corresponds to the
observation of the maximum DAE_C6 ΦUV and Φvis (Figures
5a−c and S16a−c). Furthermore, the ITIC/DAE_C6 FET
could maintain a reasonably good on/off ratio exceeding 35,
even after a continuous switching operation of 100 cycles,
which is a decrease of ∼30% from the initial on/off ratio. The
comparison in the photoswitching performance of our FET
devices with recently reported works with various molecular
switches is summarized in Table S7.
Theoretically, the ITIC/DAE FET electrical fatigue
resistance is dependent on not only the molecular photo-
stability of DAE against bi-product generation but also the
Q.Y. for photocyclization/photocycloreversion. This is be-
cause, in the case of low Q.Y., the cumulative proportion of
isomers whose conformation has not adequately changed is
continuously increased under continuous photoswitching.
Therefore, the success of DAE_C6 for electrical fatigue
resistance can be attributed to its rather high Q.Y. for both
photocyclization and photocycloreversion as compared to all
other previously reported DAEs.4,7,21,26
ITIC within the BHJ films with the presence of bulkier DAE
molecules, which allows more free volume for photo-
cycloreversion. Because of such synergistic advantages of
DAE_C6, we believe that a very successful photo-controlled
electrical switching behavior in FETs could be achieved.
Electrical fatigue resistance up to 100 repetitive cycles of
programmed UV−visible exposure has never been demon-
strated with any kind of molecular switches.
CONCLUSIONS
■
This study attempted to secure a strategic molecular approach
to maximize the switching behavior of DAEs when mixed with
organic π-conjugated semiconductors. Biphenyl substituted
DAE derivatives with different alkyl chain lengths were
successfully designed and synthesized and were employed as
ITIC FET photoswitches. Systematic photophysical, surface
energy, structural, and morphological analyses demonstrated
that (1) longer alkyl chain DAEs can be helpful in deriving
better miscibility with ITIC due to the suppressed
intermolecular interaction between the DAEs and the more
similar surface characteristics and (2) longer alkyl chain DAEs
can allow more free volume for conformational change of
themselves by more efficiently inhibiting the crystalline
ordering of ITIC. Thanks to these synergistic effects of
DAE_C6, a high on/off ratio exceeding 50 and an exceptional
electrical fatigue resistance, up to 100 repetitive cycles, were
demonstrated. We believe that this work can be a milestone
highlighting the importance of molecular compatibility
between the molecular switch and the semiconductor to
maximize the photoswitching performances.
As summarized in the photophysics section, the conversion
ratio and both the ΦUV and the Φvis increased as the DAE alkyl
length was increased. The increasing conversion ratio and ΦUV
of the DAEs with longer alkyl chains in the BHJ films with
ITIC can be explained by the decreased intermolecular
interaction between the DAE molecules due to steric
hindrance of the bulky alkyl chain and also by the decreased
surface energy difference between DAE and ITIC. Addition-
ally, as suggested in the structural/morphological analyses
section, the increasing Φvis of the DAEs with longer alkyl
chains can be attributed to a more amorphous ordering of
EXPERIMENTAL SECTION
■
Single Crystal Studies. To know the molecular conformation
and photoreactivity in crystal state of the newly synthesized DAE, X-
ray crystallographic analysis was carried out.59 DAE_C6 was
deliberately selected for crystal studies as it showed highest miscibility
with ITIC and better FET performance. Plate-shaped DAE_C6-o
crystals were obtained by recrystallization from chloroform and
acetonitrile mixture. Figure S7a shows the crystal structure of an
open-ring isomer. The distance between reactive carbon was found to
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be 3.6 Å which is suitable for photocyclization as reported earlier.60−63
For yielding a closed-ring isomer in crystal state, the mounted
DAE_C6-o sample was irradiated with UV light, but there was no
change in the crystal structure; this could be due to the strong π−π
interaction in the biphenyl ring of DAE_C6 which makes it difficult
for reactive carbon and thienyl sulfur to detort their positions to
generate the closed-ring isomer. Thus, to obtain the crystal structure
in the closed form, the DAE_C6-o solution in chloroform and
acetonitrile mixture was continuously irradiated with UV light
overnight. Interestingly, a needle-type DAE_C6-c crystal was
obtained. The obtained crystal structure for the closed-ring isomer
is shown in Figure S7b, in which the distance between reactive carbon
was only 1.5 Å which suggests that DAE_C6 is crystallized in the
closed form. Crystallographic data obtained for the abovementioned
crystals are shown in Table S3. Molecular packing for open- and
closed-ring isomers is shown in Figure S7c,d, respectively. The crystal
data files of DAE_C6-o and DAE_C6-c were deposited into the
Cambridge Crystallographic Data Centre (CCDC) and assigned
numbers 2038757 and 2038758, respectively (Supporting Informa-
tion).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.0c04219.
■
sı
Synthesis and characterization details, Q.Y. estimation
method, X-ray crystallography, OFET data, surface
analysis, GIXD, DSC, and AFM (PDF)
Crystallographic information for DAE_C6-o and
DAE_C6-c (ZIP)
AUTHOR INFORMATION
Corresponding Author
■
Dae Sung Chung − Department of Chemical Engineering,
Pohang University of Science & Technology (POSTECH),
Pohang 37673, Republic of Korea; orcid.org/0000-0003-
1313-8298; Email: dchung@postech.ac.kr
Authors
Device Fabrication. Highly n-doped Si++/SiO2 (100 nm) wafers
were used as substrates and gate. The FET geometry is a bottom
contact/bottom gate with a W/L ratio of 1580 (channel length = 5
μm). The interdigitated Ti/Au source and drain electrodes are
prepatterned on the substrates. The prepatterned substrates were
cleaned sequentially with a detergent (Mucasol), deionized water,
acetone, and 2-propanol by sonication and dried with nitrogen flow.
Dried substrates were treated with a UV−ozone cleaner to remove
various contaminants on the surface. Then, the substrates were
immersed in a mixture of OTS (0.3 mL) and toluene (60 mL) for 30
min, followed by annealing at 120 °C for 1 h. The OTS-treated
substrates were rinsed with toluene and dried with nitrogen flow. The
solution of ITIC/DAEs was spin coated on the OTS-treated
prepatterned substrates at 2000 rpm for 1 min.
Syed Zahid Hassan − Department of Chemical Engineering,
Pohang University of Science & Technology (POSTECH),
Pohang 37673, Republic of Korea
Seong Hoon Yu − Department of Chemical Engineering,
Pohang University of Science & Technology (POSTECH),
Pohang 37673, Republic of Korea
Chan So − Department of Chemical Engineering, Pohang
University of Science & Technology (POSTECH), Pohang
37673, Republic of Korea
Dohyun Moon − Department of Beamline, Pohang
Accelerator Laboratory, Pohang 37673, Republic of Korea;
orcid.org/0000-0002-6903-0270
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemmater.0c04219
1
Measurements. H NMR and 13C NMR spectra were recorded
with a Bruker ADVANCE-400 spectrometer which operated at 400
1
MHz for H nuclei, 100 MHz for 13C nuclei, and 376.5 MHz for 19F
Author Contributions
§S.Z.H. and S.H.Y. contributed equally to this work.
nuclei and are internally referenced to residual solvent signals.
Chemical shifts are reported in parts per million (ppm). Differential
scanning calorimetry (DSC) was conducted under nitrogen on a TA
Instrument (PerkinElmer DSC 4000). The sample was heated at 10
°C/min from 30 to 300 °C. UV−vis absorption spectra were
measured by UV 1650PC spectrophotometer.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
To analyze the surface of the films, contact angle measurements
and AFM (Park Systems, XE-150) were conducted. The PLS-II 2D
and 3C beamlines at the Pohang Accelerator Laboratory (PAL) in
Korea was used for confirmation of the crystalline profiles. All the
films were manufactured in the same manner as that of the FETs were
fabricated. CV was conducted using an IviumStat instrument and
conducted at a scan rate of 50 mV s−1 at room temperature under
nitrogen, with 0.1 M tetrabutylammonium hexafluorophosphate in
acetonitrile as the electrolyte; each compound was deposited onto the
working electrode from a chloroform solution. All CV measurements
were carried out with a conventional three-electrode configuration
employing a glassy carbon electrode as the working electrode,
saturated calomel electrode as the reference electrode, and Pt wire as
the counter electrode. The electrical characteristics of the FETs were
measured using a parameter analyzer (Keithley 4200A) in a nitrogen-
filled glovebox at room temperature. To observe the optical switching
behavior, a laser with a wavelength of 375 nm for the on state and a
laser with a wavelength of 520 nm for the off state were set up inside
the nitrogen-filled glovebox. Charge carrier mobilities were calculated
using the equation ΔIDS = (WCiμ/2L) × (VGS − Vth)2, where IDS is
the source-drain current, Ci is the capacitance per unit area of the
dielectric layer, μ represents mobility, W and L are the channel width
and length, and VGS and Vth are the gate voltage and the threshold
voltage, respectively.57,58
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (No. 2020R1A4A1019455 and No.
2020M3F3A2A03082631).
ABBREVIATIONS
■
DAE, diarylethene; ITIC, 3,9-bis(2-methylene-(3-(1,1-dicya-
nomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-
dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene;
GIXD, grazing incidence X-ray diffraction; FET, field-effect
transistor; OSC, organic semiconductor; Q.Y., quantum yield;
DSC, differential scanning calorimetry; NMR, nuclear
magnetic resonance; HOMO, the highest occupied molecular
orbital; LUMO, the lowest unoccupied molecular orbital
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